There is continuous research in the pulp and paper industry aimed at improving efficiencies in the various aspects of pulp and paper processes. The bleaching process is one aspect which has received ongoing attention.
The present invention is concerned with improvements in the process for improving brightness in mechanical and ultra-high yield pulps, such as groundwood pulp (GW), thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), and alkaline peroxide mechanical pulp (APMP). The processes of interest accomplish the bleaching utilizing hydrogen peroxide in an alkaline environment.
The efficiency of such processes depends on various factors, a very important one of which is the full utilization of the hydrogen peroxide. The objective is to obtain the dual efficiency of the improved brightness and reduced chemical usage that comes from increased utilization.
It is known that chemicals present in the pulp slurry will result in decomposition of a part of the hydrogen peroxide. This is particularly the case in the presence of transition metal ions. The hydrogen peroxide decomposition obviously reduces potential bleaching power, but also affects brightness by causing the formation of new chromophores. Manganese is known to be the most harmful transition metal species in this regard.
It is common to utilize stabilizers and sequestering agents to reduce the peroxide decomposition.
Notwithstanding the use of these methods, the peroxide decomposition has been an ongoing problem.
Against this background the present invention provides a process in which the peroxide decomposition is reduced and brightness enhanced relative to known processes. This process is carried out in the context of conventional peroxide bleaching processes.